1. Field
This invention relates to systems of drainage, specifically those directed to accumulations of ground water, rainfall and snow melt. More particularly, this invention is directed to systems for stabilizing hillsides through means of internal drainage thus preventing or remedying landslides or similar occurrences of said hillsides. Furthermore, the invention relates to the restoration of the stability of the hillside.
2. State of the Art
Landslides cause millions of dollars of damage and threaten hundreds of lives each year in various parts of the world. Excess water accumulation is one of the major causes of landslides and mud flows. Various methods for addressing this problem have been attempted over the years. Landslides, especially those which are water or fluid related, result when a substantially downslope directioned force, i.e. driving force, acting on the material constituting the hill face exceeds the substantially upslope directioned force, i.e. resistant force, acting to support that material in its initial position. This relationship between the driving force and resistant force may be expressed mathematically as: EQU F.sub.S =Resistant Force/Driving Force
where F.sub.S is defined as the factor of safety. When F.sub.S achieves a value less than one (1), a landslide potentiality is produced. The resistant force is composed of the forces of cohesion between the particles of the hillside material composition and the forces resulting from friction between the soil particles. The driving force is primarily a result of the force of gravity acting on the mass of material constituting the hill face material.
When the hill face material is permitted to become water soaked or saturated, the factor of safety is lowered in value. Excess water adversely affects the factor of safety in two different ways;
1. The mass of the hill face material is increased due to the added mass of the water, thereby increasing the magnitude of the driving force.
2. The resistant force is diminished in magnitude as cohesion and the frictional coefficient are both reduced in value.
One method of preventing landslides, especially those which are water or fluid related, is to attempt to limit the water concentration in the hillside material to a level in which the factor of safety as defined is equal to or greater than one (1).
In areas where excess water is the main cause of landslides, conventional horizontal-type drains are presently used to stabilize hillsides. These horizontal drains have been used since as early as the 1930's to remedy this hazardous phenomena. Horizontal drains are installed normal to the surface of the hillside, i.e. parallel to the groundwater flow nets. Since the pipes are laid parallel to the underground flow nets, they are not hydraulically efficient in removing the ground water and maintaining the slope stability. Such horizontal drains are typically small in cross-section, measuring typically in the range of 1/2" to 2" in diameter. The pipes are typically perforated and jetted normally into the hill face. In the conventional practice, no filter is used for such horizontal drains, and the perforations being small, necessarily are very susceptible to ferrous oxide clogging. Furthermore, the pipes clog up rapidly due to the small amount of silt entering the pipe. These conditions result in a limited durability as well as a relatively short use life for horizontal drains.
Since the horizontal drains are installed normal to the face of the potential landslide site, it is dangerous to effect their construction while the slide is occurring. Typically, horizontal drain construction must either await the stabilization of the hillside or be installed prior to a slide occurrence.
Horizontal drainage systems are also very dependent upon the particular soil or geological formation of the hillside. In heavy clay formations, since the permeability of the soil is very low, horizontal drains have limited effectiveness. In hillsides composed of fractured rocks where the cracks are randomly scattered, small cross-sectional pipe drains fail to intercept all the cracks in the ground water flow. Therefore, they also have limited effectiveness in dewatering the hillside. In massive semipermeable formations, when heavy precipitation occurs, horizontal drains are not able to remove penetrated water fast enough. This results in a rapid mud flow and/or landslide.
Layered soil formations, wherein the silt and sand are embedded in clay layers, also frustrate the operation of horizontal drains. Oftimes, these drains are unable to satisfactorily convey water from the various layers. Typically the problem is avoided by layering the drains in every possible lenses of permeable strata. Understandably, this procedure increases the cost of the project tremendously.
The installation of horizontal drains typically involves the use of a fluid medium to remove the bored particles produced in drilling the housing for the drain. In non-cohesive soils; e.g., sandy soils, silt loam soils or silty soils, when fluid is introduced to carry away the bored particles, the fluid may flow between the pipe and the wall of the bored hole. This fluid flow causes washouts and/or cave-ins often resulting in the collapsing of the bored hole.
In practice, the limited effectiveness of conventional horizontal drains in depressing the phreatic water table (or removing excess water) to maintain slope stability has necessitated the supplementation of those drains with other geotechnical structures. Typical geotechnical structures of this type may include retaining walls, buttresses, and similar constructions. These constructions are usually installed at the surface of a slide area. The necessity of supplementing the drains with such structures often escalates the cost of the drainage system beyond feasible economics.